A thyristor, sometimes also referred to as silicon controlled rectifier (SCR), is a switching device which can be turned on in a forward direction, i.e. when being forward biased, by supplying a positive gate trigger current pulse to a gate terminal. The thyristor is then said to be in a forward conducting state or on-state in which a current can flow in a forward direction from an anode to a cathode. On the other hand, the thyristor can also be in a forward blocking state, also referred to as off-state, meaning that a high positive voltage in the forward direction can be blocked. In a reverse direction opposite to the forward direction, the thyristor cannot be turned on. A thyristor may be reverse blocking, which means that it can block at least approximately the same voltage in the reverse direction as in the forward blocking state, or asymmetric, which means that it has virtually no blocking capability in the reverse direction. Since phase control applications commonly require reverse blocking capabilities, a phase control thyristor (PCT) is typically reverse blocking.
In FIG. 1 there is schematically shown a cross-section of a known thyristor 100. The thyristor comprises a semiconductor wafer, in which a thyristor structure comprising four semiconductor layers having alternating conductivity types, i.e. an n-p-n-p layer stack structure is formed. In an order from a cathode side 102 to an anode side 104 of the thyristor 100, the thyristor structure comprises an n+-doped cathode emitter layer 106, a p-doped base layer 108, an n−-doped base layer 110, and a p-doped anode layer 112. The n+-doped cathode emitter layer 106 is electrically contacted by a cathode metallization 114 formed on a cathode side surface of the semiconductor wafer to adjoin said n+-doped cathode emitter layer 106. The p-doped anode layer 112 is electrically contacted by an anode metallization 116 formed on an anode side surface of the semiconductor wafer to adjoin said p-doped anode layer 112. The p-doped base layer 108 is electrically contacted by a gate metallization 118 formed on the cathode side surface of the semiconductor wafer to adjoin said p-doped base layer 108.
A contact region between the n+-doped cathode emitter layer 106 and the cathode metallization 114 will be referred to as a cathode region, and a contact region between the p-doped base layer 108 and the gate metallization 118 will be referred to as a gate region.
When a positive voltage or forward voltage below the breakdown voltage VBO of the thyristor is applied between the anode metallization 116 and the cathode metallization 114, the thyristor 100 may be switched between the forward blocking state and the forward conducting state by supplying a gate trigger current pulse to the gate metallization 118. As long as no gate trigger current pulse is supplied to the gate metallization 118 the thyristor will remain in the blocking state. However, when the thyristor 100 is triggered by supplying a gate trigger current pulse to the gate 118, electrons will be injected from the cathode metallization 114, flow to the anode where they will lead to hole injection, and an electron-hole plasma will form in the p-doped base layer 108 and n−-doped base layer 110 which may switch the thyristor 100 into the forward conducting state. The forward conducting state may be maintained as long as the forward voltage is applied and will be stopped when the forward voltage applied between anode metallization 116 and cathode metallization 114 is switched off or changed to a reverse voltage. Upon applying a reverse, negative voltage between the anode metallization 116 and the cathode metallization 114, the thyristor 100 goes into a reverse blocking state and may be switched to the forward conducting state by again applying a forward voltage and another gate trigger current pulse. To obtain a full blocking state of the thyristor 100, the reverse voltage has to be applied for a certain duration called quiescence time tq such that the electron-hole plasma previously injected may disappear due to recombination processes, thereby re-enabling the forward blocking capacity of the device.
To trigger thyristor 100 shown in FIG. 1, a substantial gate current is required. A known measure to facilitate triggering of a thyristor is the integration of an auxiliary thyristor 120 together with a main thyristor 126 as in the thyristor 100′ shown in FIG. 2. The auxiliary thyristor 120 is also often referred to as pilot thyristor. An anode metallization 116′, p-doped anode layer 112′, an n−-doped base layer 110′ and a p-doped base layer 108′ of the thyristor 100′ are all shared by the auxiliary thyristor 120 and the main thyristor 126 as shown in FIG. 2. The auxiliary thyristor 120 comprises a gate metallization referred to as auxiliary gate metallization 130, which contacts the p-doped base layer 108′ in the region of the auxiliary thyristor 120. The auxiliary thyristor 120 also comprises an n+-doped emitter layer referred to as auxiliary n+-doped emitter layer 122. The auxiliary n+-doped emitter layer 122 is contacted by a cathode metallization of the auxiliary thyristor 120, which is referred to as auxiliary cathode metallization 124.
The auxiliary cathode metallization 124 is internally connected to the gate metallization of the main thyristor 126, which is referred to as main gate metallization 118. The main gate metallization 118 contacts the underlying p-doped base layer 108′ in a region of the main thyristor 126. The contact region between the p-doped base layer 108′ in the region of the main thyristor 126 and the main gate metallization 118 is again referred to as gate region. Preferably, a single, contiguous metallization serves as both, as the auxiliary cathode metallization 124 and as the main gate metallization 118. An n+-doped emitter layer 106 is comprised in the main thyristor 126 and is contacted by the cathode metallization 114 of the main thyristor 126, wherein a contact region between the n+-doped emitter layer 106 and the cathode metallization 114 in the region of the main thyristor 126 is again referred to as cathode region. Typically, the auxiliary cathode metallization 124 is not accessible from outside of the thyristor 100′, i.e. no terminal exists which would allow for a direct electric connection from the outside to the auxiliary cathode metallization 124.
Small area devices can be triggered properly by a relative moderate current applied to a small gate region in the center of the device. For large-area devices of similar gate design, a significantly higher current would be required. To improve the turn-on behavior of large area devices it is known from WO 2011/161097 A2 to distribute the auxiliary thyristor structure over the whole thyristor area, thus accelerating the spread conducting region during turn-on. This reduces turn-on losses and allows higher di/dt ratings as compared to simple central-gate structures.
For high power applications, thyristors have been developed based on circular semiconductor wafers having a diameter of e.g. 4 or 5 inches. However, advanced thyristor applications require even larger thyristor designs based e.g. on 6 inch wafers. It has been observed that for such large thyristor designs, it may not be sufficient to simply scale-up previous smaller thyristor designs. With increasing thyristor diameter, further effects may gain influence on thyristor operation. For example, a larger thyristor for higher nominal current with equivalent forward blocking capacity or turn-on characteristics as well as cooling characteristics during thyristor operation may not simply be achieved by proportionally scaling thyristor dimensions.
A thyristor 100′ as described above with a homogeneously n+-doped cathode emitter layer 106 as shown in FIG. 2 may be very sensitive to transients with positive voltage variations dV/dt, which may give rise to so called dynamic voltage triggering, which is caused by a charging current occurring during build-up of a depletion layer in n−-doped base layer 110′, which thus forms a drift region. Said charging current is amplified in a partial transistor formed by the emitter, base and drift layers of the thyristor 100′. Without impeding the forward characteristics significantly, this disadvantage may be mitigated by distributing a plurality of discrete emitter shorts 128 across the cathode region. The main purpose of the emitter shorts 128 is to allow for removal of a leakage current which occurs during the forward blocking state of the thyristor 100′, and which may lead to unintentional turning on of the thyristor. The emitter shorts 128 are formed by small through holes or vias in the cathode emitter layer 106 through which the p-doped base layer 108 may reach the cathode side surface 102 metallized with the cathode metallization 114 as shown in FIG. 3. The p-doped regions with missing n+-doping on the cathode side 102 thus formed are sometimes also referred to as cathode emitter shorts or cathode shorts as they may short-circuit the cathode junction. The emitter shorts 128 may form an ohmic short-circuit across a junction between p-doped base layer 108′ and n+-doped cathode emitter layer 106, and may conduct a significant portion of the current at low current densities, i.e. in all phases where forward blocking is required.
From EP 0 002 840 A1 there is known a thyristor exhibiting improved maximum current rise rates as a result of the relocation of the ignition front from the edge of the cathode emitter zone to inner cathode emitter areas. This relocation is effected by providing a relatively light doping of the anode zone beneath the thyristor gate and cathode emitter edge, and a relatively higher anode zone doping opposite and outside of the cathode edge, while not applying an anode electrode metal coating to the lightly doped area of the anode zone. The thyristor utilizes cathode emitter short circuit rings arranged such that the ignition front which occurs at thyristor triggering bypasses the short circuit ring immediately adjacent the cathode emitter edge, thereby increasing the thyristor voltage rise velocity, dU/dt.
From JP S53 92391 U and from JP S54 46488 A there is respectively known a thyristor device with emitter shorts, wherein no emitter shorts are provided in longitudinal areas extending in a direction away from a gate contact.
From JP S54 46488 A there is known thyristor device comprising emitter shorts, wherein the emitter shorts are arranged around the gate contact at a smaller pitch than in the remaining emitter contact area.
According to WO 2011/161097 A2 an emitter shorts pattern of a thyristor should be as uniform and homogenous as possible, ideally with a constant density of shorts, as shown in FIG. 4 in a partial top view, over the whole cathode region, and all subregions thereof, in particular in cathode regions close to the gate structure to achieve a high lateral plasma spread velocity and a high maximum current change dI/dt.
The shorting pattern controls the spread of plasma in lateral direction. The quality of shorting pattern design reflects in the relevant dynamic parameters like critical rate of rise of forward voltage dVDM/dt and critical rate of rise of reverse voltage dVRM/dt, circuit commutated recovery time tq, etc. It also impacts static parameters like gate non-trigger current IGD, gate trigger current IGT, on-state voltage VT, etc. It also strongly affects the overall reliability of a thyristor.